The author of a controversial book on Donald Trump’s White House has defended his reporting, saying that he “absolutely” stands by everything he wrote and that the US president has no credibility.

Michael Wolff, who spent months in the White House while researching the book, was responding to the president’s claims that it was “full of lies”.

Mr Wolff told NBC’s Today show that Mr Trump’s staff “all say he is like a child”.

The book has now gone on sale early.

Mr Trump’s lawyers had tried to block publication of Fire and Fury: Inside the Trump White House, alleging it contained many falsehoods.

But Mr Wolff said the president had “less credibility than perhaps anybody who has ever walked on Earth at this point”.

He added that White House staff described the president as childlike because “he has the need for immediate gratification. It’s all about him… This man does not read, does not listen. He’s like a pinball just shooting off the sides”.

The book cites former top aide Steve Bannon as describing a meeting at Trump Tower in New York between a Russian lawyer and Trump election campaign officials, including Mr Trump’s son Donald Jr, as “treasonous”.

Both Mr Trump Jr and his father strongly deny any collusion took place.

It also portrays Mr Trump as being surprised at winning the presidency.

Mr Wolff said it was “extraordinary” that the president of the US would try to stop publication of his book, a move that “the CEO of a mid-sized company” would not attempt.

Mr Trump said he had not given Mr Wolff access to the White House nor spoken to him for the book.

Mr Wolff responded: “What was I doing there if he didn’t want me to be there? I absolutely spoke to the president… It was not off the record.”

Asked if attempts to block the book’s publication, and the attendant publicity, had helped sales, he smiled and said: “Where do I send the box of chocolates?”

I authorized Zero access to White House (actually turned him down many times) for author of phony book! I never spoke to him for book. Full of lies, misrepresentations and sources that don’t exist. Look at this guy’s past and watch what happens to him and Sloppy Steve!

Billionaire conservative donor Rebekah Mercer, a Breitbart investor who had backed Mr Bannon financially, also cut ties with the former strategist, saying: “I support President Trump and the platform upon which he was elected”.